1. Field of the Invention
The present invention relates, in general, to an organic, optical data storage medium which is recordable, readable and erasable by using laser beam and, more particularly, to an organic, optical recording medium with high data storage density, high data rates and long data archival capabilities, useful as a medium for recording a variety of information or pictures.
2. Description of the Prior Art
The variety and amount of information has increased at an explosive rate in today's information-intensive society. Such an information increase requires recording media to be higher in data storage capacity or density, data rate, and to be faster in operation.
Currently, the practical or commercial techniques for recording data are based substantially on magnetic storage technology. In general, the data are stored on magnetic media, such as video tapes, audio tapes, floppy discs and the like, on which information is recorded depending on the direction of magnetization of magnetic substances in the magnetic medium.
While the magnetic storage technology is commercially successful and advantageous, a recording technique known generically as optical recordings has been and continues to be considered a very promising alternative for storage data, as a recording media with higher capacity is demanded according to the enormous amount of information resulting from the society development.
A magneto-optic recording medium comprises a recording film magnetizable in the direction perpendicular to the plane of the film itself, in contrast to the conventional magnetic medium. In addition, the coercive force of a magneto-optical medium, which is a measurement of strength maintaining the previous direction of magnetization, is about 5 to 10 times as high as that of a magnetic medium. Accordingly, using a magneto-optic recording medium, it is very difficult to change the previous direction of magnetization with an external magnetic field.
The recording of information on a magneto-optic recording film is effected by first focusing a modulated laser beam at a point approximately 1 .mu.m in diameter on the surface of the film. The laser beam power should be sufficient to heat the film locally, for example, to the Curie point temperature of the film. In this state, the direction of magnetization can be changed with an external magnetic field, so as to record information on the film according to the direction.
When the information is recorded by this method, the storage unit of information comes to be reduced to approximately 1 .mu.m or less in size. Accordingly, the recording density of magneto-optical medium is 10 to 1,000 times greater than that of conventional magnetic recording medium. In addition, the magneto-optical medium employs a non-contact reproducing method, so that magneto-optical recording potentially has significant advantages over magnetic recording, including easier data preservation and longer data archival capability.
However, there have historically been many disadvantages in producing the magneto-optic recording media used in such method. For example, heavy metals are commonly used as magnetic substances and a vacuum deposition or sputtering apparatus is required.
Many attempts have been made to solve such disadvantages. One of the attempts is to develop an organic optical recording material. The organic optical recording material may be grouped into (1) a write-once/read-many (hereinafter, referred as "WORM") type, and (2) a rewritable (hereinafter, "RW") type on the basis of the erasability of the material.
On the WORM type material, only readout of information is possible after recording data once, whereas, on the RW type material, erasure of the data is also possible after recording.
WORM type medium is manufactured, as disclosed in Japanese Patent Laid-Open Publication Nos. Sho. 57-46362, 58-197088, 59-5096 and 63-179792, by coating laser-absorbent dye admixtured with polymer on a reflective layer to form a recording layer and overcoating a protective layer on the recording layer. In this optical recording system, to write a data bit, a laser beam is focused on a very small spot of the recording layer, for example, approximately 1 .mu.m or less in diameter, to generate sufficient heat in the laser-absorbent dye, which heat decomposes the polymer to form a pit. The reproduction of the recorded information is effected by using the difference of the reflectivity according to the presence of the pit. Since, in WORM type material, the recorded portion is in a polymer-decomposed state, it is impossible to rerecord data on the polymer-decomposed portion after erasing the information.
A RW type material has been vigorously researched and the direction of the research proceeds to a heat mode using a light as heat or to a photon mode using a photon of light.
In the heat mode, the recording or reproducing of information is effected by optical change generated when a recording laser beam is irradiated to a localized area of the recording layer to bring out melting, vaporization, thermal deformation, thermal transfer and the like.
As an optical recording medium employed in such heat mode, there have been energetically studied two optical mediums, wherein one medium employs TbFe, CdFe, TbFeCo and the like, taking advantage of the Paraday effect and the Kerr effect. The other heat mode optical recording medium employs inorganic metals represented as Te, such as TeOX, Te--Ge, Te--Ge--Sb, and Te--Ge--Sb--Ti. However, since this heat mode optical medium employs a material harmful to human body and it is produced, employing a sputtering method, the stability of the medium is low and it is difficult to carry out the production process. What is worse, the heat mode optical medium shows low recording sensibility and is problematic in the stability of recorded state and the write-over capability on high speed erasure.
Therefore, there is demanded a nontoxic, low-priced medium capable of erasing data in a high speed and of showing high recording sensibility and stability.
To develop a material for satisfying these properties, a variety of methods employing a nontoxic, low-cost, organic polymeric material have been suggested.
For example, a method utilizing a thermoplastic resin and a far infrared ray-absorbent dye has been disclosed in Japanese Patent No. 5848245 and a method using a polymer blend has been proposed in U.S. Pat. No. 4,722,595. However, these methods are problematic in recording sensitivity and recording/erasure repetition.
A variety of methods utilizing the phase transfer of liquid crystal have been proposed in Japanese Patent Laid-Open Publication Nos. Sho. 59-10930, 60-114823 and 60-166481, and U.S. Pat. No. 4,904,066. However, since an electrode is employed in these patents, the structure of the medium is complicated and there are disadvantages in recording sensitivity, responsibility and reliability.
As polymeric liquid crystal materials used in optical recording, main chain type liquid crystal polymers have been reported in Japanese Patent Nos. 6128004 and 62175939 and side chain type liquid crystal polymers have been described in German Patent No. 3500838. However, they can not be put into practical use since their recording sensitivity, contrast and repetitive erasability are unsatisfactory and the speed of response is slow.
To improve the responsibility of liquid crystal, a photoisomeric method has been advanced in Japanese Patent No. 6398852, by which a photochromic molecule, such as azobenzene, is bonded to a side chain of a polypeptide having a photochromic liquid crystal layer, using the relation between photoreaction and liquid crystal. However, this method also shows some problems in contrast, repetitive erasability and data archival capability and thus, is not industrially available.
In the meantime, the optical recording method according to the photon mode has attracted attention by virtue of its high sensibility and high speed erasure. As an optical recording material, there have been photochromic materials using spiropyran compounds in Japanese Patent Publication No. Sho. 61-17037, fulgid or indigo in Japanese Patent Publication No. Sho. 61-128244.
However, while these materials have superior such properties as high sensitivity and high speed erasure, they are inferior in stability and repetitive erasability in a color developing state due to their poor light resistance. In addition, since an ultra violet ray and a visible ray are, in general, used as a recording light and an erasing light, respectively, in photochromic compounds, it is difficult to stably store data and a reverse reaction is apt to occur during a photochromic reaction.
Owing to the aforementioned problems, the optical recording medium has not been rapidly developed in spite of its superior properties, such as high data storage density and high speed.